1. Field of the Invention
The present invention generally relates to semiconductor memory devices, and particularly relates to a semiconductor memory device that performs a refresh operation for retaining data.
2. Description of the Related Art
The memory capacity required in mobile terminals such as cellular phones tends to increase as the functions of the terminals become increasingly sophisticated. Conventionally, SRAMs (static random access memories) have been used as memory devices in mobile terminals. In recent years, however, DRAMs (dynamic random access memories) have begun to be used for the purpose of providing a large memory capacity. What becomes a problem at such an attempt is the usable time of batteries in the mobile terminals.
SRAMs do not consume power for the purpose of retaining data. DRAMs, on the other hand, perform refresh operations at constant intervals to retain data, and consume power to some extent even in the standby state. Namely, even when mobile terminals are not in use, the holding of data in memory consumes electric power, resulting in the usable time of backup batteries being shortened.
In order to obviate this problem, the number of refresh operations in the standby state may be reduced so as to suppress power consumption. For example, the data retention time of DRAMs tends to increase as temperature drops. When temperature is low, therefore, the refresh intervals may be lengthened compared to when temperature is high, thereby reducing the number of refresh operations.
If the refresh intervals are controlled in response to the temperature detected by a temperature sensor in a straightforward manner, a problem as described in the following may arise.
A data retention time is short in the standby state if temperature is high. Refresh operations are thus performed at short intervals. If temperature suddenly drops from this condition, the memory cells that have been exposed to high temperature are automatically switched to long-interval refresh operations despite the fact that such memory cells need short-interval refresh operations. As a result, a refresh operation ends up failing to be completed within the time period necessary for data retention, resulting in a serious problem such as loss of data.
Accordingly, there is a need for a semiconductor memory device that is configured to adjust refresh intervals in response to temperature, and that can properly retain data even when a sudden temperature change occurs.